The present invention relates to a vibration isolating mount device for mounting a power plant on an automobile, and particularly, belongs to the technical field of a structure thereof in a case where a mechanism for limiting oscillation in a rolling direction of the power plant is installed integrally in a single body.
In a front engine-front drive (FF) type vehicle, for example, generally a power plant has been conventionally mounted on the vehicle body with the length direction (a direction along which a crankshaft extends) thereof aligned in the width direction of the vehicle body so that both end portions thereof are elastically supported by the body side frames located on both sides, left and right, of the engine room (a so-called traverse mount type).
As such a traverse mount type, main stream thereof is an inertia main axis mount scheme (or a torque roll axis scheme) in which main mounts on both ends, left and right, of the power plant are disposed in the proximity of a roll inertial main axis (hereinafter referred to simply as a roll axis) thereof, which is disclosed, for example, in Patent literature 1 (FP No. 2736008 A). This is because a dynamic spring performance around the roll axis is made comparatively softer with a secured support stiffness of two mounts on the left and right sides, respectively, supporting almost all the static load of the power plant, thereby enabling idle vibrations to be effectively reduced.
With the dynamic spring performance around the roll axis softer to such an extent, for example, when a driving output (torque) of the engine greatly alters as in rapid acceleration or rapid deceleration in the case of the torque roll axis mount, the power plant in the entirety greatly rolls around the roll axis by the reaction force (torque); therefore, plural mounts for limiting the rolling are usually provided before and after the power plant in addition to the main mounts on the left and right sides.
In this connection, another mount scheme is disclosed, for example, in Patent literature 2 (DP No. 4209613 A) in which supporting points for a power plant of main mounts on the left and right sides not only are set more higher than a roll axis to thereby oscillatably support the power plant in the entirety but also dispose a torque rod for limiting the oscillation at the lower end portion thereof so as to connect the power plant to the vehicle body side (this scheme is hereinafter referred to as a pendulum mount scheme).
Since in the pendulum mount scheme, main mounts on the left and right sides are farther away from a roll axis than in the general torque axis mount scheme, a force in the vehicle body longitudinal direction acts directly on the main mounts in company with rolling of the power plant; in other words, the rolling is limited by the mounts themselves. In consideration of this, according to the mount scheme described in Patent literature 2, torque rods are provided to the main mounts on the left and right mounts, respectively, in addition to an independent torque rod attached at the lower end of the power plant, thereby causing a force in the vehicle body longitudinal direction to be received.
In a case where torque rods are provided to the main mounts in such a way as well, however, the number of parts constituting the mounts increases correspondingly to thereby raise the number of assembly man-days; therefore, leading to a problem to cause a lot of cost up. This is because both ends in the front and rear sides of each of the torque rods are supported so as to be revolvable around respective horizontal axes so that a vertical motion of the corresponding mount is not hindered by attachment of the torque rod and the torque rod is connected to the corresponding mount with a rubber bush so that vibration is not transmitted to the mount.
In connection with this aspect, there has been known various kinds of stopper mechanisms capable limiting displacement in a mount with a simpler structure. In a mount scheme described, for example, in Patent literatures 3 and 4 (JP No. 2003-184939 A and JP No. 2002-257182 A), stopper rubber portions are provided so as to protrude on both sides before and after a member connected to the power plant side and the stopper rubber portions are brought into contact with members of the vehicle body side which the stopper rubber portions face in the vehicle body longitudinal direction, thereby preventing displacement beyond the members of the vehicle body side.
Since, if the stopper rubber portions are, in such a way, brought into contact with the vehicle body, an adverse possibility arises that vibrations of the power plant are transmitted to the vehicle body side through the stopper rubber portion, stopper mechanisms described in Patent literatures 5 and 6 (JP No. 61-92329 A and JP No. 60-02541 B) each are equipped with the stopper rubber portion having protruded stripes at the distal end thereof or the stopper rubber portion having a hollow in the interior thereof, thereby reducing a stiffness thereof partly. With partial reduction in the stiffness, no increase in spring constant of the mount occurs suddenly at the same time as the action of the stopper (S1), for example, as shown with a solid line (I) of FIG. 7A and increase in spring constant at an initial stage of the action of a spring occurs in a relatively mild manner, thereby enabling vibrations to be absorbed, as shown with a broken line (II) in the same figure.
The conventional stopper mechanisms (disclosed in Patent literatures 3 to 6 and the like) each, however, have no function that an increase in dynamic spring constant in the vertical direction can be restricted to the lowest level without hindering upward and downward motions of a mount by receiving and absorbing a force in the vehicle body longitudinal direction acting on the mount as a torque rod acts, so the mechanisms each cannot be a substitute for the torque rod.
That is, in the stopper mechanisms of Patent literatures 3 and 4, as described above, since elastic deformation of compressed rubber is greatly limited not only in the vehicle body longitudinal direction but also in the vertical direction in a state where the stopper rubber portion is in contact with a member of the vehicle body to receive a pushing force in the vehicle body longitudinal direction, the member of the vehicle body is restricted in motion thereof by the rubber stopper portion relative to the member of the power plant. For this reason, dynamic spring constants of the mount in the entirety in the longitudinal direction and vertical direction of the vehicle body increase suddenly at the same time as the action of the stopper, thereby degrading vibration isolating performance (see the solid lines shown in the graphs of FIGS. 7A and 7B).
In acceleration of an automobile, for example, a power plant in the entirety, in some case, has great vibrations in the vertical direction because of unbalance in reciprocating inertia force or the like of the engine, when the power plant is inclined by a driving reaction force to thereby cause a stopper to act and to degrade vibration isolating performance of mounts suddenly, which leads to an inconvenience that vibrations in the vertical direction accompanying the acceleration operation propagates into the vehicle compartment to generate a loud surrounding sound confined therein.
Even if stiffness of the stopper rubber portion is partly reduced to cope with such an inconvenience to thereby cause rubber to be elastically deformed with comparative ease at the initial stage in the action of a stopper, as shown in Patent literature 5 and 6 described above (see the broken line in the graph of FIG. 7A), such a feature works only so as to slow down degradation of vibration isolating performance of a mount in company with action of the stopper, and does not work such that increase in dynamic spring constant in the vertical direction of the mount as a whole is suppressed to thereby enable vibrations in the vertical direction to be sufficiently absorbed while a load in the vehicle body longitudinal direction is received with certainty, as a torque rod works.